Thermal power plants and nuclear power plants use a steam turbine to generate electric power. Components of the steam turbine such as a turbine rotor are exposed to an environment subject to corrosion. A crack due to stress corrosion cracking, corrosion fatigue, or the like occurs in a zone on which stress is concentrated such as a blade groove of a turbine rotor disc. The crack grows as an operation continues, and if it is left untouched, a blade is finally disengaged from the turbine rotor disc to destroy other parts. Thus, the power plant conducts regular tests to check whether a crack has occurred in each part of the turbine rotor disc and regularly recognize a growth situation thereof.
As shown in FIG. 8, a turbine rotor R used in a steam turbine includes a plurality of stages of turbine rotor discs (hereinafter sometimes simply referred to as “rotor discs”) 101 axially arranged at predetermined intervals on a rotor shaft 100. Each rotor disc 101 has a turbine blade 106 mounted to a blade groove 104 formed in an outer peripheral portion.
In the turbine rotor R described above, if a defect portion such as a crack is created in an outer peripheral portion such as the blade groove 104 and therearound of the rotor disc 101, the turbine rotor R needs to be removed from the steam turbine and conveyed to a repair plant for repair.
To repair the rotor disc by build-up welding, Tungsten Inert Gas (TIG) welding or submerged arc welding may be adopted. Conventionally, the TIG welding at a low welding speed has been more likely to be adopted to prevent grain coarsening at a weld heat affected zone. However, considering welding conditions has allowed application of the submerged arc welding at a high welding speed.
Among them, Patent Literature 1 discloses that a member that prevents flux from dropping from a surface to be welded is arranged close to a rotor disc, and thus flux can be stably deposited on the surface to be welded through a welding step to stably repair a rotor disc by submerged arc welding even if the rotor disc has a small diameter.